In printing machines, such as printers, copiers, facsimile machines, multi-function machines and the like, a substrate is conveyed through various stations of the apparatus. For instance, in a digital copier, the substrate or sheet bearing the image to be copied may be mechanically conveyed across a platen in proximity to an imaging apparatus. In addition, sheets may be mechanically extracted from a supply and fed through image transfer stations and finishing stations in the digital copier. One exemplary machine is depicted schematically in FIG. 1. This machine 10, which may be a primary print processing device or a finishing station, directs the substrate received through an inlet chute into a processing station 12. The finished substrate exits the machine through an outlet chute 15 into a collection element 17, for instance.
In many such machines the substrate may pass along multiple paths that are generally defined by chutes and baffles, such as the baffle assembly 14 shown in FIG. 1. The substrate is typically propelled along these paths by nip roller assemblies, such as the nip rollers 20 and 21, which include a driven roller and one or more idler rollers that “pinch” or “nip” the sheet therebetween. The nip roller assemblies are situated at pre-determined intervals along each substrate path, with the intervals generally corresponding to the smallest size sheet being fed through the path. While the idler rollers do not drive the sheet directly, they are important in providing the nip force normal to the direction of travel of the sheet to ensure non-slip feeding or transport of the sheet and to help ensure that the substrate travels straight along the path without skewing or translating laterally. These functions of the idler roller are particularly accentuated in a long transport path where accumulated alignment errors may cause jams, or may require expensive re-registration stations to re-align the sheet within the path.
It is necessary that the idler rollers be freely rotatable as well as slightly vertically movable to accommodate different substrate thicknesses passing through the nip roll. This vertical degree of freedom is also necessary to account for variable deformations of the drive roller or to adjust for wear of the nip roller components. One known system for allowing the idler roller to vertically “float” is depicted in FIG. 2. The Substrate passes between a drive roller D and an idler roller I. The axle A of the idler roller I is supported within a slot formed in a frame M. In this known system, a one or more extension springs E supported by the frame M straddles a bushing supporting the axle A of the idler roller and exert a downward force on the roller.
While this system may be acceptable for many nip rollers in a transport path, in some machines variable nip force is required. For example, in some finishing machines a sheet is initially allowed to slip through the nip roller assembly in one direction (which may be accomplished by using a nip force significantly lower than that of the downstream nip), but a high nip force is required to drive the sheet in a reverse direction. This approach is commonly used to buckle the trailing end of the sheet for the purpose of registering the trailing edge against a backstop. Certain prior systems rely upon the spring, such as the extension spring E, or a torsion spring, to provide the necessary force. However, in these approaches, the spring rates are usually very high in order to apply a sufficiently large force for a small deflection of the spring. As a result, the applied force is widely variable and difficult to control. Ultimately, this prior approach requires very tight tolerances for the components of the nip roller assembly.